1. Field of the Invention
The present invention relates to an apparatus for controlling the transfer of power to a rotary element without the use of wires. It is useful in electroplating apparatus and more particularly, for recharging batteries which supply power to rotating elements in a rotary plater.
2. Description of the Background Art
There are many applications for devices with rotary elements which require some supply of electric current or charge to portions of the rotating elements. Electroplating is one of these applications. Electroplating has been used in the manufacture of thin film electronic components and magnetic read/write heads for disk drives. The quality of devices manufactured by this process is very sensitive to the uniformity of the metal films that are deposited. Uniformity of thickness is crucial as well as uniformity of composition of the deposit.
Using magnetic head manufacture as an example, the process is carried out by starting with a substrate, which has a conductive seed metal layer deposited on a planar surface (referred to as the "top" even though in most plating steps, this surface will actually face downward into the plating solution), the deposition of the seed metal usually having been accomplished by a vacuum deposition or sputtering process. A photo-resist stencil is placed on top of this conductive metal layer. The substrate is then connected to an electrical power supply, with the substrate becoming the cathode (usually) with respect to the plating bath when voltage is applied. An anode (for most purposes) is created in the form of a plate connected to a positive terminal of the power supply, submerged in a plating bath. The substrate is also placed to be in contact with the plating bath, usually just barely interfacing with the surface of the bath so that the face surface (top) of the substrate engages the liquid but the edges and reverse surface of the substrate do not. Positively charged metal ions (cations) are drawn to the negatively charged substrate, and a layer of metal accumulates on the exposed seed metal surface, until the desired thickness has been achieved.
As mentioned above, a primary concern is the uniformity of thickness of the metal deposit. In an effort to obtain more uniformity, one kind of electroplating device spins the substrate to minimize the effects of localized variation in ion concentrations in the electroplating solution. Although this generally improves uniformity, there may still be problems of non-uniformity with radial displacement across the substrate. The biggest factor in this non-uniformity is generally the drop off in electric field strength found near an edge or corner of the plating target area of the substrate. A particular element from the central portion of the plating target area which is bounded on all sides by identical elements can be presumed to have a generally uniform electrical field applied over its extent. An element at an edge of the plating target area, however, which does not have identical neighboring elements on all sides, can be expected to have an electric field which differs in strength and configuration from that found in interior elements. This element attracts metal ions differently and thus can result in a non-uniform deposition layer in these regions.
In an effort to compensate for these "edge effects" it has become common practice to include what is called a "thieving ring" around the perimeter of the substrate. This thieving ring is also negatively charged and serves to restore uniformity to the electric field at the edges of the substrate by providing similarly charged neighboring elements. The outer perimeter of the thieving ring has edge effects of its own, but is treated as a sacrificial element where the uniformity of metal deposition is unimportant.
Early thieving rings were constructed in one piece so that a uniform field was supplied around the entire perimeter of the substrate. However, it has been found that by segmenting the ring, and applying differing currents to the different segments, finer control of the electric field configuration could be achieved. This is especially useful when processing substrates and chip arrays on substrates which are not completely circular. For processing of substrates and arrays which are perhaps rectangular, or otherwise configured to have corners or protrusions, it is very beneficial to be able to adjust the charge on those specific thieving ring segments near those corners or protrusions in order to achieve more uniform field strength and thus achieve greater uniformity of deposition.
One problem in using a thieving ring having a number of segments is that power must be supplied to each segment independently from the others in order to optimize the electrical field. Therefore, at least one wire is required to be attached to each segment. In current practice, it is common to have sixteen segments, each with its own power supply wire. This number may increase to an even larger number of segments if finer control of the field configuration is desired. In addition, there are four wires required to supply power to the substrate itself. Thus, there may be a minimum of twenty wires involved. This number is increased when it may be necessary to sense the voltage and current on each supply wire in order to ensure that the proper fields are generated. In dealing with this great number of wires, the problem is compounded by the rotary nature of the cathode. Rotary connectors have an intrinsic problem relating to the difficulty of maintaining reliable electrical connections with a rotating element. Such connections are very prone to noise and friction effects. Any noise generated in the connection can be detrimental to the uniformity of the electric field and thus the uniformity of the deposition layer. When trying to maintain twenty to forty such rotary electrical connections, the odds of failure are thus greatly increased.
Power can be transferred to the cathode and the thieving ring segments through the shaft of the motor which drives the rotation of the cathode. A controller such as a microprocessor can then be mounted on the rotary substrate structure to control the current distribution to the thieving ring segments. Although it is theoretically possible to share part of the plating current from the plating power supply to power the microprocessor through the motor shaft as well, in practice, it has been found to be difficult to implement. Microprocessors and associated electronics are sensitive to plating supply fluctuations, which may stay at zero current for long periods of time. Thus an appropriate voltage divider storage and regulation circuit to supply power to the microprocessor is necessarily bulky, generates heat, and may interfere with the electroplater's performance. Further, the power supply to the substrate is typically a very highly sensitive constant current supply and it is desirable to avoid introducing variables which might affect the field generated about the substrate.
To address these problems, a battery can be mounted on the rotary cathode structure to supply power to the microprocessor. However, the rotary cathode, which may run at speeds of 1-50 RPM during plating, can run at as much as 5,000 RPM, when the plate is spun to dry it. The forces generated during such high-speed rotation can make mechanical fastening of batteries too costly, too heavy or too unreliable. Consequently, batteries have typically been hand-soldered in place, which necessitates a lengthy replacement operation when the batteries become depleted. Batteries which are rechargeable in place are therefore preferred, but again, there is the problem of transferring power across a rotating interface to charge them.
Many prior patents have dealt with the problems of improving uniformity of plating by using a rotary cathode plater.
U.S. Pat. No. 5,785,826 to Greenspan describes an apparatus for electroforming in which the entire cathode head is mounted on a lead screw by which the distance between the cathode and anode can be adjusted, in order to control uniformity of deposition.
U.S. Pat. No. 5,628,892 to Kawashima describes an electroplating apparatus and method using an arcuate segmented anode. Electroplated metal foil is produced by depositing metal ions from an electroplating bath on the surface of a rotating cathode drum. This device produces metal foil with minimized thickness variations.
U.S. Pat. No. 5,472,592 to Lowery discloses an electroplating apparatus in which a cathode plate both rotates and revolves around the inner surface of an electroplating tank. Electrical connections are made through an elaborate series of concentric contact bushings which form three electrically isolated conducting paths to evenly distribute current to a substrate.
U.S. Pat. No. 4,304,641 to Grandia describes an electroplating cell having a rotary cathode using a thieving ring. A flow-through jet plate is used to provide differential flow distribution for the plating solution. Variable resistors, to keep the current constant, control electrical current to the cathode and ring.
Each of these patented devices uses either a rotating or revolving cathode in an effort to produce improved uniformity of plating, but none of these address the problem of providing reliable and noise-free supply of power to a microprocessor and associated electronics without using wires.
It is, therefore, an object of the present invention to provide an improved method and apparatus for reliably supplying power to a rotary element such as a microprocessor mounted on the cathode structure of a rotary plater. Other objects and advantages will become apparent from the following disclosure.